Soil contamination is common in the United States. Once the soil becomes contaminated, precipitation may infiltrate though the soil and carry contaminants downward into groundwater. Because groundwater is the primary source of drinking water for about fifty percent (50%) of the population of the United States and contaminants may negatively affect human health and the environment, mitigation of contaminants in the subsurface is important.
Contaminants may be categorized as volatile organic compounds (VOCs), semivolatile organic compounds (SVOCs), fuels, inorganics (including radioactive substances), or explosives. The physical properties of these contaminants dictate the processes that may be effective in their treatment. For example, the nature of the contaminant structure will help determine how biodegradable a contaminant may be. More polar (and more soluble) molecules tend to be more readily biodegradable. Higher molecular weight molecules tend to be more difficult to degrade. Fewer chlorine atoms make a molecule easier to degrade, and a high degree of branching make a molecule harder to degrade. Physical properties such as a contaminant's octanol water coefficient (Kow), for an organic compound, will give an indication of the contaminant's mobility in an aquifer and its adsorption potential.
Over the years, a number of techniques have been developed to address subsurface contamination including groundwater pump and treat systems (P&T), air sparging (AS) systems, permeable reactive barriers (PRBs), chemical oxidation (ChemOx), and in-situ biological treatment systems (IBT). The remedial approach selected at any given site varies depending on the contaminants, their distribution, site conditions, and the treatment objectives. As with any remediation approach, selected treatment systems have their advantages and disadvantages. However, beyond the application issues, these systems often have limited effectiveness, and after implementation, residual contamination may remain at a site for decades or longer.
A number of factors can limit the accessibility of contaminants for treatment. Contaminants in an aquifer partition and equilibrate between the soil, aqueous, and nonaqueous liquid (or pure contaminant) phases. Following distribution and equilibration, attempts to remove contaminants are often hindered by the typically slow kinetics of desorption. Also, many organic compounds such as naphthalene have low solubilities and an affinity for the organic matter in the soil. In many cases these contaminants have more mass in the soil phase then in the aqueous phase. In addition, contaminants may migrate by diffusion into tighter portions of the soil fraction and be less easily removed with groundwater or addressed through the liquid phase.
A result of limiting site conditions and processes is that technologies such as P&T are often ineffective. Other technologies, such as ChemOx, may be cost effective for treatment of source areas, but are typically too costly to apply widely at sites and due to the limitations above often leaves residual contamination. Many other processes, such as air sparging, also have limited effectiveness due to aquifer properties and conditions that limit biological degradation of contaminants. PRBs can take extended periods of time to clean up at a contamination site because they rely on the slow leaching of contaminants from the solid to liquid phase and then treat the liquid phase contamination before it migrates off-site. This can result in unreasonably long time frames for site cleanup, as well as long term operation and maintenance costs.
Monitored natural attenuation (MNA) is a widely accepted remedial option for site closure that relies on sorption and naturally occurring biodegradation to attenuate contaminants. Application of MNA typically involves source control to remove “hot spots” of contamination followed by monitoring the site to demonstrate that the contaminant plume is stable or shrinking. However, MNA has its limits. Both the biodegradation and sorption component of the remedy may not be supported by conditions in the aquifer. In some instances, natural degradation of contaminants may be limited due unfavorable environments for growth. In other cases, subsurface soils may have a very low organic content, measured in fractions of percents, and as a result, contaminant migration is not slowed significantly by the sorption reaction. If these limiting conditions could be corrected, the natural degradation rate would increase and there would be more time for natural degradation to reduce contaminant levels. This would significantly enhance the overall subsurface environment for remediation, make MNA more widely applicable, and other remedial options significantly more effective.
What is needed are treatment technologies that more effectively addresses contaminants in an aquifer formation by resolving at least some of the limiting conditions in aquifers. Such a system could also be used to enhance the performance of a variety of treatment systems. The present invention offers such a solution, by treating contaminants in an aquifer by modifying aquifer properties and making them more favorable for contaminant treatment.
Adsorbent materials have unique properties that allow them, once emplaced, to treat contaminants and enhance aquifer properties for treatment generally. Sorbent materials, such as activated carbon (AC) for example, have a very high sorption capacity due to their very large surface area and numerous and variable pore sizes. These properties allow such materials to sorb significant quantities of a wide variety of contaminants. It is also known that microorganisms find activated carbon to be a very favorable environment on which to grow. Emplacing materials like activated carbon in a dispersed fashion within a contaminated medium, like an aquifer, with nutrients and/or electron acceptors/donors if necessary, would provide exceptionally favorable conditions for contaminant treatment. Sorption would attract and accumulate contaminants on the carbon, where microbes could then grow and degrade the adsorbed contaminants. Activated carbon would also, by increasing the sorption reaction, significantly slow the migration or movement of the contaminant in the aquifer. Activated carbon amendment alone could slow or stop the migration of contaminants off site, make MNA more widely applicable, and make remediation using almost any treatment technology, implemented in concert with activated carbon, more effective. AC is also considered an inert (i.e., a stable material that is not dissolved by its intended reaction with a contaminant in the subsurface) and non-biodegradable substance. As such, by selecting an appropriately sized AC particle, it can be permanently introduced into an aquifer, as discussed in greater detail infra.
Activated carbon is typically manufactured in a process that involves dehydration and carbonization followed by activation. The primary result of this process is a material of primary carbon. Burning off decomposition products to expose pores allows for subsequent widening and development of pores during activation. Some coal based carbons are converted to a fine powder prior to processing into activated carbon. By necessity, the carbon is reconstituted using a binder. Other composite particles of carbon and other materials will also contain a binder. The binder, while typically stable may not be as stable or provide as strong a bond as carbon made without a binder. Failure of the binder, while unlikely, may result in the release of very fine particles that may migrate with the contaminant attached. Activated carbon without binders is essentially elemental carbon and ash, and as such is inert, and stable. It is the object of the instant invention to emplace the carbon as permanently as practicable, as such activated carbon without binders is preferred. If activated carbon with binders is used, the conditions of use and permanence of the binders should be considered. Those practiced in the art will appreciate that different types of activated carbons are available. Carbons may be classified as H-type (hydrophobic), or L-type (hydrophyllic). The hydrophobic/hydrophyllic properties result from the different processes used to manufacture each carbon type. The H-type carbon is generally preferred for permanent emplacement, as the carbon will not migrate under typical aquifer conditions. The L-type carbons are generally preferred for injection/extraction as they will tend to move more readily with injected and extracted water. In addition, the pore space of the activated carbon can be matched to the size of the contaminant being absorbed, as discussed in greater detail infra.
As used herein, “adsorption” is defined as the two dimensional accumulation of one or more contaminants on the internal and external surface of a solid, whether adsorbed permanently or reversibly, where the adsorbed compound or substance partitions between the solid and liquid phases in an equilibrium reaction; “absorption” is defined as taking one or more contaminants into the three dimensional volume of the solid through or as through pores or interstices, and allows for diffusion to internal sites of the solid. It is believed that the three dimensional absorption, which includes diffusion, may be the cause of sometimes slow equilibration and desorption times; “sorption” and “sorb” are defined as the process in which one substance (sorbent) takes up, accumulates, or holds another by either absorption or adsorption; “sorbent material” is defined as any substance or a variety of substances that have the ability to attract and accumulate various contaminants at their surface and/or within the substance(s) through sorption, and/or are sorbent relative to the contaminant; “activated carbon” is defined as a solid substance that is very porous, has a large surface area, and is produced from a wide variety of organic materials with a high carbon content including but not limited to wood, coal, peat, lignin, nut shells, bamboo, bagasse (sugar cane pulp), sawdust, corn cob, lignite, bone and petroleum residues. These precursors are manufactured into activated carbon by a process of dehydration, carbonization, and activation. Activated carbon may be powdered, granular, extruded, or a composite of different materials such as iron and carbon, affixed to other materials, reconstituted into pellets or cylinders, or may be activated carbon fibers (ACFs) with activated coating on glass fibers not limited to rayon, acrylic, polyvinyl alcohol, polymers such as cellulose or polyacrylicnitrile, and phenolic materials. Activated carbon is characterized by a high surface area, typically ranging from about five hundred square meters per gram (500 m2/g) to about one thousand seven hundred square meters per gram(1,700 m2/g) but can range up to about two thousand five hundred square meters per gram (2,500 m2/g), a highly porous structure, and the ability to absorb, accumulate, and concentrate large quantities of organic molecules and inorganic molecules. Activated carbon may also act as a catalyst in oxidizing or other reactions, or a support for catalysts such as precious metals catalysts. Activated carbon may range in size from the micron range up to larger than ten (10) millimeters; “treat,” “treating,” and “treatment” are defined as sorbing a contaminant onto or within a sorbent so that the contaminant can be reduced, naturally broken down  (i.e., biological degradation), transformed, and/or slowed from migrating at or beyond a contaminated site so that the release of the contaminant from the site is often so slight so as to be below governmentally set regulatory levels, criterion, or treatment goals; “attenuation” refers to the process by which a compound is reduced in concentration over time, through absorption, adsorption, degradation, dilution, and/or transformation, either alone or in combination, and includes reducing the migration rate of one or more contaminants within a contaminated site to allow natural degradation to occur; an “aquifer” is defined as any subsurface soil bearing water; a “contaminant” is defined as any organic chemical substance or substances that may be toxic or have otherwise adverse effects on human health and/or the environment, and may be present above regulatory criteria, and includes chemicals such as volatile organic compounds (VOCs), a broad group of halogenated and nohalogenated compounds that are volatile, semivolatile organic compounds (SVOCs) a broad group of halogenated, nohalogenated, polycyclic aromatic hydrocarbon, pesticide, dioxin and furan, and polychlorinated biphenyl compounds that are semi volatile, fuels or petroleum products such as gasoline, diesel, and jet fuels and their constituent compounds, typically found in soil and groundwater or other media at contaminated sites; “injection point” refers to borings, injection rods, wells (vertical and horizontal), infiltration galleries, or any other subsurface interface, or any other means which is/are used to emplace or introduce the sorbent material and carrier into the contaminated medium in a dispersed fashion, whether created for testing the contaminated site or another purpose, or created expressly for the purpose of practicing the instant invention; an “injection” is defined as introducing a slurry consisting of sorbent material and a carrier into the subsurface; a “carrier” is defined as water, guar gum, air, nitrogen, gases, aerosols, or other amendments such as coagulants, polymers, polyelectrolytes, hydrogen release compounds and substances (HCs, which are more applicable to chlorinated compounds, including lactate and lactic acids) or oxygen release compounds and substances (OCs, which are more applicable to aromatics and hydrocarbons, including slow release peroxides), chemical oxidants (Fentons, modified Fentons, potassium permanganate, permanganates, ozone, peroxides, hydrogen peroxide, etc.), mixes thereof, and other liquids or substances that may be used to make a slurry with the sorbent material to facilitate introducing and dispersing the sorbent material into the contaminated media, adequately keep the sorbent material suspended and flowable so that it may be injected into or emplaced in the subsurface or contaminated media, and/or help stabilize the sorbent material in place; “coating(s)” are defined as microencapsulations, nanoencapsulations, coatings, timed release coatings, and controlled release coatings including but not limited to surfactants (sometimes sugar based), natural polymers (including but not limited to starches and cellulose), proteins (including but not limited to soy, corn, gelatin, collagen, polyglutamic acid, casein, and chitosan), vegetable gums (including but not limited to gum Arabic, guar, locust bean, pectin), natural minerals (including but not limited to clays), sulfur, waxes, and synthetic polymers; “amendments” are defined as nutrients, micro nutrients, co-substrates, electron donors, electron acceptors, microorganisms, bacteria, yeast, enzymes, HC, OC, lactate, surfactants, molasses, guar gum, food grade oils, ethanol, acetate, gases, methane, oxygen, nitrogen, hydrogen, ozone, aerosols, hydrogen peroxide, materials that cause oxygen to be released from aquifer materials, buffers, acids (such as phosphoric acid), bases, lime, phosphates, ammonia, nitrogen species such as nitrate, minerals, with the above substances sometimes coated for timed release or microencapsulated, or other substances, and mixes thereof added to the sorbent material or sorbent material slurry to enhance contaminant removal and treatment of the contaminated site; “contaminated media” refers to soil, groundwater, sediment, or sludge that contain contaminants; “permanent” does not mean ad infinitum, but refers to long-lasting without significant change, not subject to erosion, and/or intended to exist for a long, indefinite period of time without regard to unforeseeable conditions, i.e., stable; and “treatment goals” refers to contaminant treatment goals, requirements, criteria, or objectives established by statute, code, or regulation, or by agreement with regulatory authorities.